As shown in FIGS. 1 and 2, a reed is mounted to a magnetic circuit by compression. As temperature changes the much-thicker magnetic circuit component, it deforms the reed into a novel shape, which causes the paddle to deflect. Additionally, any slip of the mounting points causes error.
The slip potential increases at high temperature, as the clamping force decreases and shear stress between the reed and the magnetic circuit increases.
The coefficient of thermal expansion (CTE) (α) of the excitation ring 13 is higher than a of the attached reed 16. As the excitation ring 13 is also considerably stronger, it will pull mounting points 18 radially, which will also cause a compression or tensile stress as the excitation ring 13 attempts to move the mounting points 18 to a smaller or larger radius.
Because the fused silica (commonly referred to as quartz) of the reed 16 is a highly elastic material, the reed 16 does not plastically deform to accommodate the metal of the excitation ring 13. Instead, some other mechanism of stress accommodation occurs, Possibilities are: a) slip of the mounting points 18; b) local yielding of metal part; and c) the rim of the reed 16 becomes an oval shape, which forces the paddle 19 out of plane. Any one of these, or a combination thereof, will cause sensor error that is made worse by temperature extremes.
FIGS. 1 and 2 illustrate an accelerometer that includes the asymmetric flexure arrangement of the present invention. The accelerometer measures acceleration along sensing axis SA, and includes stator, reed, and mounting members 18. The reed is held between mounting member 18 and stator, and has a coil positioned on its upper surface. The excitation ring (e-ring) comprises stator 13, magnet and pole piece. The e-ring is shaped so that the coil occupies a comparatively narrow gat between pole piece and stator 13, to provide the force balancing function well known to those skilled in the art.
The reed has an overall disk-like shape, and includes annular support ring and paddle connected to one another via a pair of flexures between Which an opening is farmed. For most of its perimeter, the paddle is separated from the support ring by a circular gap. Raised mounting pads 18 are located at approximately equally spaced positions around support ring.